“Prevention Through Design” a Front Line Defense for Mitigating
Power Distribution Arc Flash Hazards
December 2017 | By Brian Schmalberger
Brian Schmalberger is an OEM segment market leader for ABB.
The risk of an electrical arc flash incident with power distribution equipment is never zero – so a healthy appreciation of risk is always a smart line of defense. Power distribution system designers know that to mitigate or minimize the impact of an arc flash incident requires the right combination of anticipating problems (both human error and stressed equipment), designing and enforcing safe practices outlined by safety and electrical standards, and employing arc mitigation technologies to reduce this dangerous and potentially deadly risk. Taken together, the practice of “Prevention Through Design,” or “PtD” provides a clear pathway toward reducing the hazards of an arc flash and promoting worker safety.
Getting Workers Away From Harm’s Way
Even with the best arc flash hazard study, well-defined operational and maintenance procedures, and advance arc flash mitigation technologies, accidents can happen. Using an uninsulated tool or dropping a tool or loose part into an energized piece of equipment such as a switchgear, motor control center or power panel enclosure can cause a catastrophic accident. Even with warning labels and the best personal protective equipment (PPE), safety protocols are not always followed. Workers get accustomed, and sometimes “casual,” about working near dangerous equipment, or fatigue or setting ambitious maintenance work schedules can lead to lax adherence of safety protocols.
Arc flash Prevention Through Design is an end-to-end approach, from process and procedures through the use of advanced operational tools and mitigation technologies, all working in in sync to provide new levels of protection. At the front line of PtD defense is keeping workers outside of hazardous working boundaries – such as “arc flash,” “shock protection,” “restricted approach,” and “prohibited approach” boundaries. When working within these areas, again, clear and well-defined safety labels on all equipment highlights the need for safe operations. Ideally, however, relocating personnel and their monitoring and maintenance tasks well outside these boundaries is always a one of the best PtD practices.
Remote Monitoring and Control Systems
Many electrical system monitoring and maintenance operations–such as reading metering functions, opening or energizing breakers or reviewing a log of condition events–can be done remotely with human machine interface (HMI) displays and touch-screen controls. These HMI units can be located tens or even hundreds of feet away from the main power equipment being served.
A typical HMI screen can be in a wall-mounted enclosure, or housed in a separate control stack located near the equipment but well outside of the hazard zone. A dedicated control stack is typically situated away from energized equipment and can be powered by a common 120V circuit. In addition to monitoring and controlling the breakers, other operations, such as checking battery status of the redundant UPS, can be done via HMI controls. Additionally, many HMI systems allow for wireless monitoring circuit breakers outside the arc flash zone via a smart phone or tablet.
One of the more dangerous maintenance or repair operations on switchgear, switchboard or motor control units is disconnecting and reconnecting power when swapping out large capacity “draw-out” circuit breakers or MCC buckets. Disconnecting, removing, reconnecting and energizing a draw-out circuit breaker can be done manually with a screw crank and turn-screw (or pinion) mechanism that racks, or draws, the unit in and out of position. The proximity of a worker when manually racking a unit can be extremely hazardous.
Remote racking systems use a remote-control racking motor and gearbox configuration that connects directly to the circuit breaker housing. Portable roll-up units can also be positioned to engage the racking pinion, allowing operators to work away from the racking operation. These remote portable units use a gear motor and enclosure, with a hand-held control attached with 30 foot cable, enabling maintenance personnel to rack low voltage breakers in and out at a distance for greater arc flash protection.
RELT – Energy-Reducing Maintenance Switch
Another key worker safety technology fundamental to PtD is the mandated use of an energy-reducing maintenance switch, which we refer to as reduced energy let-through, or RELT. This technology lowers the energy and duration of an arc flash event. RELT technology is essentially a secondary instantaneous protection function that enables faster instantaneous trip protection. Before starting maintenance operations that involve higher arc flash risk, personnel can use a RELT switch to temporarily adjust the sensitivity of a trip device from 1.5 times up to 15 times the normal protection device rating. This adjustment effectively helps to control the duration, and severity, of an arc flash incident. The incorporation of this energy-reducing maintenance switch into low- and medium-voltage power protection devices became a mandate with the adoption of the 2017 National Electric Code (NEC) 240.87 standard.
The RELT feature should be initiated before working on equipment and involves workers both activating this alternative setting and getting positive feedback that the system is activated. The activation and signal function, transmitted via a 24Vdc signal, can be located on an equipment-mounted control or remotely with a switch outside of the arc flash boundary. For RELT to be most effective, standard operating procedures should be developed and followed so that all personnel are aware of when the trip unit sensitivity is changing.
Connected Power Protection Makes Monitoring and Maintenance Safer
Smart power distribution systems, employing a combination of sensors, communications-enabled circuit protection, software and remote monitoring and diagnostic capabilities are also creating new layers of safety to help mitigate the impact of an arc flash and to protect workers. A range of new circuit protection and trip units, including our EntelliGuard* and GuardEon* line of circuit breakers, are embedded with communications capabilities to connect with monitoring, remote operations and diagnostics tools. For example, smart power systems can monitor system performance enabling proactive failure alerts or alarms, or generate data for predictive maintenance.
These new communications features give operations teams new levels of visibility–and planning and reaction capacity–that create new operational efficiencies.
The combined arc flash mitigation and protection tools that make up a smart and safe power distribution system all come together under the “Prevention Through Design” concept to provide new levels of protection for workers.
Additional Reference and Sources
Use of HMI and Remote Controls
Remote Racking Systems for Arc Protection